1. Field of the Invention
The present invention relates to technology that exposes an object, such as a monocrystal substrate for a semiconductor wafer, or a glass substrate for a liquid crystal display (LCD) using, for example, light in the extreme ultraviolet range of 200 nm to 10 nm, or in the X-ray range.
2. Description of the Related Art
Conventionally, when manufacturing a minute semiconductor device, such as a logical circuit or a semiconductor memory using photolithography (exposure) technology, a reduction projection exposure apparatus is used that projects and transfers a circuit pattern formed on a reticle or a mask (hereafter, referred to as “an original”) onto a wafer, or the like, through a projection optical system.
With this reduction projection exposure apparatus, the smallest size (resolution) that can be transferred is in proportion to the wavelength of the light used for exposure, and is in inverse proportion to the numerical aperture (NA) of the projection optical system. Accordingly, as the wavelength becomes shorter, the resolution increases. For this reason, the wavelengths of exposure light have become increasingly shorter, in response to demands for more minute semiconductor devices, in recent years. Thus, the wavelengths of ultraviolet light that are used are becoming shorter, such as light of an extra-high pressure mercury lamp (i-ray (wavelength approximately 365 nm)), a KrF excimer laser (wavelength approximately 248 nm), and an ArF excimer laser (wavelength approximately 193 nm).
However, the minuteness of semiconductor devices is increasing at a rapid pace, and there is a limit to the manufacture thereof with lithography using ultraviolet light. Therefore, in order to efficiently transfer an extremely minute circuit pattern of 0.1 μm or below, EUV exposure apparatuses are being developed that use light in an extreme ultraviolet region of wavelengths, in the range of about 10 to 15 nm, which are shorter than ultraviolet light wavelengths.
Since the absorption of light by substances becomes extremely large along with the shortening of exposure light wavelengths, it is difficult to use a refractive element, i.e., a lens, which utilizes refraction of light that can be used with visible light or ultraviolet light. Further, no glass exists that can be used in the wavelength region of EUV light, and, therefore, a refraction optical system is used that forms an optical system with only reflector elements, i.e., mirror members (for example, multilayer mirrors), that utilize light reflection.
A mirror member does not reflect all of the exposure light, and absorbs 30% or more of the exposure light. The absorbed exposure light turns into partial heat and deforms the surface shape of the mirror to cause a deterioration in the optical performance (particularly, image formation performance) thereof. Consequently, the mirrors are made of low thermal expansion glass with a small coefficient of linear expansion, such as, for example, a coefficient of linear expansion of 10 ppb, in order to reduce changes in the mirror shape caused by temperature changes.
Zerodur™ (manufactured by SHOTT) is a typical example of the low thermal expansion glass described above. In the case of Zerodur™, a temperature (zero crossing temperature) exists at which the coefficient of thermal expansion thereof becomes zero at about room temperature, and thus, it is considered that Zerodur™ can be used at about that temperature.
Since an EUV exposure apparatus is used for exposure of circuit patterns of 0.1 μm or less, the accuracy of a line-width is extremely critical, and only a deformation of about 0.1 nm or less is allowed in the surface shape of the mirror. Accordingly, even if the coefficient of linear expansion of the mirror is 10 ppb, the temperature gradually rises, and the shape of the mirror surface changes. For example, assuming the thickness of the mirror to be 50 mm, the shape of the mirror surface will change by 0.1 nm as the result of a temperature increase of 0.2° C.
To overcome this problem, because the EUV exposure apparatus is disposed in a vacuum, various methods have been proposed, such as a method involving heat transfer or radiation from a cooling plate that is provided so as to surround the mirrors of the optical system.
However, the heat generated by an EUV exposure apparatus is not just exposure heat. Typical heat sources are a vacuum pump that is attached to the vacuum chamber (external heat with respect to the vacuum chamber), or a sensor or actuator that is disposed inside an illumination optical system or a projection optical system, or the like (internal heat with respect to the vacuum chamber).
Japanese Patent Application Laid-Open No. 2002-124461 proposes a method for solving this problem.
The method disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2002-124461 will now be described using FIG. 5.
In FIG. 5, EUV light PB, which is emitted from a light source LA, is condensed on a mask MA through a radiation system IL, and is then condensed on a substrate W through a projection system PL. Reference characters MT and WT denote tables for scanning the mask MA and the substrate W, respectively.
These optical systems are housed in a vacuum chamber VC that has a vacuum pump VP, and are configured so that radiant heat of the vacuum pump VP is deflected by use of a heat deflector TB with a low coefficient of radiant heat, so that the heat does not affect the projection system PL, or the like. Further, the systems are configured so as to surround heat that is generated from the projection system PL or substrate table WT, or the like, with a plate TE with a high coefficient of radiant heat, which is a sealed member.
As described above, according to an exposure apparatus that requires a vacuum atmosphere, radiant heat from the vacuum pump VP is deflected with the plate TB, or the like, that has a low coefficient of radiant heat, and heat from other heat sources is absorbed with the plate TE, or the like, that has a high coefficient of radiant heat.
However, since it is difficult for the deflected heat or absorbed heat to be released to outside of the vacuum chamber VC, because of the vacuum atmosphere, the temperature inside the vacuum chamber VC rises. The low thermal expansion glass (Zerodur™) that is used inside the vacuum atmosphere is used at about a zero crossing temperature, as described above. Therefore, there is a problem that even if the temperature in the vacuum chamber VC is kept constant, the temperature around the glass rises above the zero crossing temperature, due to the influence of radiant heat from the vacuum pump VP, and the like, and the glass cannot be used in the ideal temperature environment.